US 8236014 B2
Methods are disclosed for the formation of arterio-venous fistula creation. Embodiments include a femoral access approach to the creation of an Aorta-caval fistula at the bifurcation of the Aorta and the Inferior Vena Cava; an apparatus for the creation, modification and maintenance of a fistula; and a method of supplying oxygenated blood to the venous circulation of a patient. The devices, systems and methods can be used to treat patients with one or more numerous ailments including chronic obstructive pulmonary disease, congestive heart failure, hypertension, hypotension, respiratory failure, pulmonary arterial hypertension, lung fibrosis and adult respiratory distress syndrome.
1. A method of creating an aorto-caval fistula in a patient at or near a bifurcation of an Aorta and an Inferior Vena Cava, between a starting vessel and a target vessel, wherein the starting vessel and the target vessel comprise the Aorta and the Inferior Vena Cava or the Inferior Vena Cava and the Aorta, respectively, comprising:
placing an arterial catheter into the Aorta superior to the aortic bifurcation;
placing a venous catheter in the Inferior Vena Cava, inferior to the fistula creation site;
introducing a crossing needle device into the starting vessel;
injecting contrast medium through one of the venous and arterial catheters into the target vessel;
advancing a crossing needle between the Aorta and Inferior Vena Cava in multiple discrete steps;
passing a first guidewire through the crossing needle into said target vessel;
confirming access of the target vessel by advancing a second guidewire through the crossing needle;
advancing an anastomotic clip delivery device over said first guidewire;
deploying a clip from the delivery device to form a side-to-side anastomosis between the Aorta and the Inferior Vena Cava.
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This application claims the benefit of prior provisional application No. 60/579,913, filed on Jun. 14, 2004, the full disclosure of which is incorporated herein by reference.
The present invention relates generally to medical devices and methods. More particularly, the present invention relates to devices and methods for creating a flow of oxygenated blood into the venous system of a patient.
Chronic obstructive pulmonary disease affects millions of patients in the United States alone. The present standard of care is oxygen therapy, which requires a patient to remain near a stationary oxygen source or carry a bulky oxygen source when away from home or a treatment facility. It is easy to appreciate that such oxygen therapy has many disadvantages.
Lung reduction surgery has recently been proposed for treating patients with chronic pulmonary disease. Such surgery, however, is not a panacea. It can be used on only a small percentage of the total patient population, requires long recovery times, and does not always provide a clear patient benefit. Even when successful, patients often continue to require supplemental oxygen therapy.
There is therefore a need for improved approaches, including both devices and methods, for treating patients suffering from chronic obstructive pulmonary disease. If would be desirable if such devices and methods were also useful for treating patients with other conditions, such as congestive heart failure, hypertension, lung fibrosis, adult respiratory distress syndrome, and the like. Such devices and methods should provide for effective therapy, preferably eliminating the need for supplemental oxygen therapy in the treatment of chronic obstructive pulmonary disease. After the procedures, such devices and methods should optionally be adjustable so that the degree of therapy is responsive to the patient's needs at any particular time. At lease some of these objectives will be met by the invention described hereinafter.
According to a first aspect of the invention, a method for creating an aorto-caval fistula in a patient is disclosed. The fistula is made at or near the bifurcation of the Aorta and the Inferior Vena Cava (IVC) between a starting vessel and a target vessel. An arterial catheter is placed in the Aorta superior to the aortic bifurcation, and a venous catheter is placed in the Vena Cava, inferior to the fistula creation site. A crossing needle device is introduced into the starting vessel, and contrast medium is injected through a catheter in the target vessel. A crossing needle of the crossing needle device is advanced from the starting vessel to the target vessel, and a guidewire is passed through the crossing needle into the target vessel. An anastomotic clip delivery device is advanced over the guidewire, and an anastomotic clip is deployed between the starting vessel and the target vessel, after which the anastomotic clip delivery device is removed.
In a preferred embodiment of the method of the present invention, the starting vessel is an artery, and the crossing needle device is inserted through an introducer placed in the left femoral artery. Prior to advancing the crossing needle, the apparatus may be positioned for support against the medial aortic wall. The crossing needle is preferably advanced at least five millimeters, and the guidewire may advance in a superior direction up the IVC toward the right atrium of the heart or in an inferior direction into the Right Iliac Vein. In an alternative, preferred embodiment, the starting vessel is a vein, and the crossing needle is advanced from vein to artery. Subsequent advancement of the guidewire is into the Aorta from the IVC.
In another preferred embodiment of the method of the present invention, blood is aspirated through the crossing needle to confirm position of the crossing needle in the target vessel. Contrast injections can be performed to alternatively or additionally confirm position of the crossing needle in the target vessel.
In yet another preferred embodiment of the method of the present invention, the crossing needle device is removed prior to creation of a blood flow path between the artery and vein. In an alternative embodiment, the flow path is created prior to removing the crossing needle device. It may be desirable to dilate or debulk material from the flow path prior to advancing the anastomotic clip delivery device. Dilation can be performed with a compliant or non-compliant balloon, and debulking can be performed with ablative devices, such as radiofrequency ablation devices, or material cutting devices, such as pull back atherectomy devices.
In yet another preferred embodiment of the method of the present invention, the anastomotic clip is deployed by advancing a core of the anastomotic clip delivery catheter. In an alternative embodiment, the anastomotic clip is deployed by retracting a sheath of the anastomotic clip delivery catheter. During deployment, it may be desirable to visualize, such as via fluoroscopy and a contrast injection, the bulge of the wall of either the artery or the vein, as contact is made between the anastomotic clip and the vessel wall. After creation of the blood flow path and/or placement of the anastomotic clip, it may be desirous to inject contrast media in the artery upstream of the fistula to confirm and assess fistula blood flow.
In yet another preferred embodiment of the method of the present invention, a dilation is performed after the anastomotic clip is deployed. The dilation may be used to plastically deform the anastomotic clip, expand the tissue surrounding the anastomotic clip, or both. In another embodiment, a second anastomotic clip is deployed such as to enhance scaffolding of the neighboring tissue, or to reduce lumen diameter or both.
In yet another preferred embodiment of the method of the present invention, a second fistula is created to increase flow from the arterial system to the venous system. In yet another preferred embodiment, a flow modification procedure is performed to modify the flow or other fistula parameter such as to increase flow rate or decrease flow rate. Flow modification procedures can be performed during the same clinical procedure as the fistula creation procedure, such as within an hour of placement of the anastomotic clip, or can be done in a subsequent clinical procedure such as more than twenty four hours after placement of the anastomotic clip. The flow modification procedure may be based on an analysis of physiologic data such as blood pressure, heart rate, cardiac output, PaO2, PaCO2, PvO2, PvCO2, PApr, PI O2, O2 saturation, mean system arterial pressure, mean system venous pressure, respiration, blood glucose, heart rate variability or other heart parameter. The analysis may also be based on specific outcome target levels such as those set by a clinician, patient disease sate or the number of flow modification procedures performed. Multiple analyses may also be performed.
In yet another preferred embodiment of the method of the present invention, the flow of the fistula is biased towards a lower than targeted flow rate such that a planned subsequent dilation can be performed after anastomotic clip placement, in order to “fine tune” or otherwise optimize the final flow conditions. The planned dilation may be based on an analysis of physiologic data.
In yet another preferred embodiment of the method of the present invention, a fistula modification procedure may be performed, pre or post deployment of the anastomotic clip. The fistula modification procedure may include one or more of: covering a portion of the anastomotic clip, applying an antibiotic agent, applying an anti-infective agent; applying an anti-proliferative agent, applying a source of light, applying a source of heat, applying a source of cooling, applying an anti-thrombotic agent and providing a dose of radiation.
In yet another preferred embodiment of the method of the present invention, a filter is used during the fistula creation procedure, such as a vena cava filter placed in the IVC, or other filter, such as an umbrella filter, placed in a vein or artery. It may be desirable to have filtration means in both a vein and an artery during one or more steps of the fistula creation procedure.
Both the foregoing general description and the following detailed description are exemplary and are intended to provide further explanation of the embodiments of the invention as claimed.
The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate various embodiments of the present invention, and, together with the description, serve to explain the principles of the invention. In the drawings:
Reference will now be made in detail to the present embodiments of the invention, examples of which are illustrated in the accompanying drawings. Wherever possible, the same reference numbers will be used throughout the drawings to refer to the same or like parts.
Shunt device 150 has been placed in a surgical or interventional procedure, or a combination of the two. In an open surgical procedure, a clinician use scalpels and other cutting means to expose the associated vessels to create fistula 110. Alternative, minimally invasive surgical procedures make use of one or more tubes, placed through small incisions in the skin, through which the clinician can pass various visualization and surgical tools to create the connection between the Aorta 130 and IVC 120. In an interventional procedure, similar to balloon angioplasty and interventional atherectomy procedures, catheter devices are placed through introducer tools into one or more vessels, and advanced through the vasculature to a specific location by guided fluoroscopy or ultrasound. Access to fistula 110 can be made superior to fistula 110 such as by way of jugular veins and arteries. In a preferred embodiment, interventional access is provided from the groin area of the patient, such as by way of Right Arterial Iliac 132 or Left Arterial Iliac 133 for access to the Aorta 130, and Right Venous Iliac 122 or Left Venous Iliac 123 for access to IVC 120.
The flow of oxygenated blood from the arterial system through fistula 110 to the venous system can provide therapeutic benefit to patients suffering from one or more diseases including but not limited to: chronic obstructive pulmonary disease, congestive heart failure, hypertension, hypotension, respiratory failure, pulmonary arterial hypertension, lung fibrosis and adult respiratory distress syndrome. The therapeutic benefit results from one or more factors as is described herebelow.
Blood returning to the right side of the heart is pumped to the lungs where is becomes oxygenated or re-oxygenated before returning to the left side of the heart to be pumped to the body's tissues via the arterial system. Blood flow experiences a resistance from all of the system vasculature, which is referred to as systemic vascular resistance (SVR). The re-circulated blood that passes through shunt device 150 bypasses the peripheral microcirculation and decreases the SVR. To achieve therapeutic benefit, a decrease of SVR of at least 5% would be desired.
Blood flows through shunt device 150 from Aorta 130 to IVC 120 because of the pressure gradient between the blood in the arterial system and the blood in the venous system. In a preferred embodiment, the flow through shunt device 150 is at least 5 ml/min. It may be desirable for shunt device 150 to self-regulate flow, or be controllable via internal or external means, as will be described in reference to subsequent figures herebelow. The flow of arterial blood into IVC 120 has cardiac, circulatory and respiratory effects. Cardiac output increases with a decrease in SVR due to the increased pressure gradient. This increase in cardiac output could benefit patients with cardiac failure or patients who suffer from low cardiac output, such as congestive heart failure patients.
Regarding respiratory effects, the oxygenated blood that mixes with the venous blood already present in IVC 120 results in a higher O2 concentration venous blood entering the right atrium of the heart. This high O2 concentration venous blood leads to an increase in the O2 concentration in arterial blood in two ways: (1) since the blood that is shunted does not have O2 extracted by tissue capillaries, the blood returning to the lungs has a higher O2 concentration after the creation of the shunt than before, and (2) the binding of O2 to the hemoglobin component of blood is more efficient with a higher Pa O2 (partial pressure of O2 in arterial plasma) resulting in increased oxygen carrying capacity. These advantageous respiratory effects could benefit patients with pulmonary arterial hypertension by lowering pulmonary arterial blood pressure, patients with heart or respiratory failure by increasing arterial oxygen concentration, or patients with chronic obstructive pulmonary disease by increasing blood oxygen concentration.
Regarding circulatory effects, another important benefit of decreasing SVR is related to the fact that the lungs regulate their blood flow according to the O2 content. An increase in the O2 content should decrease the pulmonary arterial blood pressure. These advantageous circulatory effects could benefit patients with hypertension by lowering systemic arterial, systolic and/or diastolic blood pressure. These cardiac, respiratory and circulatory effects could also benefit numerous other patients with circulatory or other diseases including but not limited to: hypotension (by increasing cardiac output), lung fibrosis, adult respiratory distress syndrome, and the like.
Various interventional techniques can be used to create a fistula at or near the bifurcation of the Aorta and the Inferior Vena Cava as was depicted in reference to
A second introducer sheath, arterial introducer 135 is placed in the groin area of the patient into the left femoral artery. Fistula creation apparatus 10, is inserted through arterial introducer 135 and advanced to a location proximate fistula creation site 111. Apparatus 10 can provide numerous functions including but not limited to: injection of contrast medium including radiographic dyes and ultrasonic medium, injection of drugs or other agents, aspiration of blood, vessel to vessel needle advancement, visualization of internal structures such as via ultrasound, fistula and/or implant dilation, fistula and/or implant contraction, tissue debulking, placement of an anastomotic clip, removal of an anastomotic clip, passage of guidewires and other small diameter devices, placement of a fistula treatment device, placement of a anastomotic clip treatment device, placement of a flow modification device, placement of a visualization device such as an intravascular ultrasound catheter and other functions. These various functions can be performed by or with the assistance of apparatus 10 through the use of functional elements integrated into apparatus 10, or separate devices which can be passed through one or more lumens accessible from the proximal end of apparatus 10.
Located at the proximal end of apparatus 10 are various knobs which are used to rotate, advance, retract, manipulate, activate, or otherwise control the slidable tubes, needles and other elements of apparatus 10. Sheath advancement knob 22 is mechanically connected to outer sheath 12 which surrounds various internal tubes, elements and lumens. In a preferred embodiment, apparatus 10 includes a visualization element, such as an ultrasound element, not shown. The visualization element can produce, through electronic or other means, a visual representation of the device and neighboring tissue. The visualization element may be an ultrasound catheter, such as a rotational or fixed array ultrasound catheter, which creates a cross-sectional image of the area surrounding the device. The ultrasound catheter can be inserted into a lumen of apparatus 10, or may be an integrated ultrasound device that has as an array of ultrasound crystals which are fixedly mounted along the distal portion of apparatus 10 and contain electronic connections that are connected to a proximal handle of apparatus 10, these connections mating with a standard ultrasonic viewing monitor. In an alternative or additional, preferred embodiment, apparatus 10 includes one or more visualization markers, such as radiographic markers or embedded agents such as barium sulfate, or ultrasonically visible markers, all not shown. These markers can be used to perform controlled advancements, retractions, rotations and other positioning of apparatus 10 during the fistula creation procedures
An internal tube, core 11 is slidingly received within outer sheath 12, and has at its distal end tip 21, which preferably has a dilating tip shape, and is atraumatic. Tip 21 is advanced to fistula site 111, by advancing apparatus 10 through arterial introducer 135. Within core 11 is another tubular device, a flexible, advanceable needle, needle 30 which has attached at its proximal end, needle advancement knob 24. Slidingly received within needle 30 is a standard interventional guidewire, guidewire 42. Needle 30 may consist of an outer protective sheath, not shown, with a flexible, advanceable needle contained within its lumen.
Tip 21 is positioned against the wall of Aorta 130 such that needle 30 can be advanced from an artery, Aorta 130, to a vein, IVC 120. In an alternative, preferred embodiment, the procedure is performed from vein to artery, such as from IVC 120 to Aorta 130. Prior to advancing needle 30 out of tip 21, radiographic dye can be injected to visualize the border of the starting vessel, Aorta 140, under fluoroscopy. An injection of contrast medium from a catheter in the target vessel can visualize the border of the target vessel walls as well. Contrast medium can be injected through the lumen in needle 30, or via a separate lumen incorporated through the length of apparatus 10.
Referring specifically to
In the artery to vein approach depicted in
Referring now to
Referring now to
Apparatus 10 may include various flow path or channel creation means, described in more detail in reference to subsequent figures. Flow path dilation elements may include the incorporation of a compliant or non-compliant balloon, with an inflation lumen and port located on the proximal end of apparatus 10, all not shown. The balloon may be integrated into a distal portion of the needle assembly, on a distal portion of outer sheath 12, on a distal portion of core 11, or on a separate tubular device advanceable through a lumen of apparatus 10, all not shown. Debulking means may include one or more of an energy ablation device, such as a radiofrequency ablation element, either monopolar or bipolar, or a cutting element similar to a pull back cutting element used in pull back atherectomy procedures. The debulking means can be advanced from the starting vessel to the target vessel to perform the debulking procedure, advanced to the target vessel first and then pulled back from the target vessel to the starting vessel to perform the debulking procedure, or both. In all flow path creation and enlargement procedures, the flow path creation and enlargement elements can be integrated into one or more components of apparatus 10 or be included in a separate tubular structure advanceable through a lumen of apparatus 10. These flow path creation and enlargement procedures are all performed over the guidewire placed from the starting vessel to the target vessel.
During retraction of apparatus 200 or one of its components, a balloon integrated on the core 211 of apparatus 200, not shown, may be inflated to help bias clip 250 in an open position during retraction. This particular embodiment may also be important if retro-peritoneal bleeding is suspected. Prior to complete retraction, a contrast medium injection from the arterial side can be used to assess blood flow through the fistula. In a preferred embodiment, guidewire 240 is not removed until proper flow and/or sufficient therapeutic benefit are confirmed. If flow is determined to be insufficient, or even too great, subsequent procedures can be employed to change the flow characteristics, such procedures described in more detail herebelow.
Apparatus 300 includes outer sheath 312 which slidingly receives one or more internal components within one or more internal lumens. Fistula treatment device 360 is a catheter device including an inner surrounding guidewire 342 such that as either fistula treatment device 360 or apparatus 300 are advanced, the advancement tracks along guidewire 342. Fistula treatment device 360 includes near its distal end, fistula treatment element 311 which has been advanced to remain within the inner diameter of clip 350 of fistula 310. Fistula treatment element 311 can take numerous forms to increase or decrease the flow through fistula 310, modify the structure of clip 350 or fistula 310, add or remove material or agents from fistula 310 or clip 350, or otherwise modify one or more characteristics or properties of clip 350 or fistula 310.
In a preferred embodiment, fistula treatment element 311 is a balloon used to dilate clip 350 and fistula 310. Clip 350 may be constructed of plastically deformable materials, either totally or partially, such that dilation will expand those materials to a greater diameter. For example, clip 350 may include self-expanding material at its ends, and plastically deformable materials at its midsection that that dilation increased the diameter of the midsection and resultant fluid pathway at one or more locations along the fistula. Dilation of a fully self-expanding clip 350 may also be appropriate to allow further expansion of clip 350 due to expansion and/or deformation of the tissue surrounding clip 350. Decrease in fistula flow can be accomplished by decreasing the diameter at one or more locations along the fistula, such as with a specialized device that pulls a portion of the midsection of clip 350 towards a smaller diameter.
In another preferred embodiment, fistula treatment element 311 may include deployment of a second implant, not shown, to increase or decrease flow properties, to provide one or more drugs or agents, or to otherwise modify the fistula and/or clip 350. Placement of a second implant may be used to enhance scaffolding, prevent bleeding, reduce lumen diameter and/or perform another function.
In another preferred embodiment, fistula treatment element 311 may include an element to perform one or more of the following functions: covering a portion of the anastomotic clip, applying an antibiotic agent, applying an anti-infective agent; applying an anti-proliferative agent, applying a source of light, applying a source of heat, applying a source of cooling, applying an anti-thrombotic agent and/or providing a dose of radiation.
Modification of fistula 310 may be performed during the same procedure that created fistula 310, such as within an hour of the creation of fistula 310 and over the same guidewire used to deploy clip 350, or the modification procedure can be performed in a subsequent procedure such as a procedure greater than twenty four hours from the creation of fistula 310. In such a subsequent procedure, a new guidewire, introduced from either Aorta 120 or IVC 130 can be manipulated using standard interventional techniques, to access the fistula and cross over to the connected vessel, either in the same direction as used in fistula creation or the opposite. Apparatus 300 can then be advanced over the vessel to vessel guidewire to perform the fistula modification or treatment procedure.
Also depicted in
Additional and/or alternative flow modification procedures can be performed to increase or decrease flow of arterial blood into the venous system. For example, creation of a second fistula between the Aorta 130 and IVC 120, not shown, or other location between an artery and a vein, can be made to increase flow. The second fistula can be created during the same procedure as the first fistula creation procedure, such as within an hour of the first fistula creation, or in a subsequent procedure such as a procedure more than twenty four hours from the creation of the first fistula. The flow modification procedure can be performed before the anastomotic clip is placed, or after it is in position between artery and vein.
In a preferred embodiment, fistula maintenance apparatus 300 of
In a preferred embodiment, a luminal diameter for the fistula and resultant flow rate is chosen to be at the lower end of a target size and/or flow rate. Subsequent to initial fistula creation, one or more parameters are measured and assessed according to steps 381, 382 and 383. The flow properties are increased, such as via the fistula maintenance apparatus 300 described in relation to
In an alternative embodiment, covered stent 370 is placed to completely cover clip 350 and fistula 310 ceasing all fistula flow. This cessation of flow may be desirous if certain adverse conditions outweighed the benefit of the fistula flow, if there was insufficient patient improvement, and/or if the need for the fistula flow has subsided. The fistula flow may be indicated to be stopped based on assessment such as an assessment performed in the methodology described in reference to
Referring now to
Surrounding core 411 is outer sheath 412, also made of flexible materials such that apparatus 400 can be advanced in its entirety, over a guidewire and through specific vasculature of human and other animal patients. Core 411 includes a reduced diameter, or step, step 414, near its distal end and clip 450 is located at this reduced diameter portion and within the diameter of outer sheath 412 such that if clip 450 is constructed of self-expanding materials, outer sheath 412 will maintain clip 450 in a compressed configuration. If core 411 is advanced, step 414 will provide sufficient force on clip 450 such as to push clip 450 out of outer sheath 412.
Referring now to
Apparatus 500 of
The apparatus 500 of
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Numerous alternatives prior to deploying anastomotic clip 550 can be employed to simplify delivery. One or more luminal tubes, such as needle sheath 531 and needle 530, can be removed or partially retracted. In alternative embodiments, one or more of these luminal tubes may include dilating tips to assist in these tubes crossing through the starting vessel and target vessel walls. One passed through, a dilating or other enlarging element incorporated into one or more of these advanceable tubes can be used to create a fluid flow channel or flow path between the vessels.
Referring now to
Referring now to
In a preferred embodiment, apparatus 500 of
Referring now to
The clip 650 of
Clip 650 may consist of a single component or multiple components. In multiple component configurations, all the components may be configured to be implanted in a single deployment or multiple deployments. After deployment, it may be desirable to remove one or more components of clip 650 at a time such as within an hour of original placement, or at a time more than twenty four hours from initial placement.
Anastomotic clip 650 may include a covering along its entirety, or include a covering limited to one or more portions along its length or diameter. Coverings may consist of a single substance, or multiple types of coverings can be used at different locations along the periphery of clip 650. Covering materials may include one or more of polytetrafluoroethylene, elastomer, nickel titanium alloy or other biocompatible material. Coverings may be porous or impermeable, and may be biodegradable. Clip 650 covering and/or other materials of construction may include or be coated with one or more agents to prevent atheroma or infection, such agents described in detail hereabove. An alternative approach to long term patency would be to have a roughened or sintered surface on the inner diameter of clip 650, in an attempt to cause clot to occur without losing fluid flow, the clot itself evolving through a healing process to the eventual creation of an endothelial layer along the inner diameter of clip 650.
Different geometric configurations of clip 650 can be utilized to maintain the arterio-venous fistula. In a preferred embodiment, geometries and materials of construction are chosen such that the radial force exerted at the mid-section is greater than the radial force exerted at either or both ends. In another preferred embodiment, the diameter of clip 650 is larger at one or both ends than at the midsection. In another preferred embodiment, the length of clip 650 when expanded is less than or equal to the largest diameter of the inner lumen. In another preferred embodiment, the length of clip 650 when expanded is less than the length when it is compressed and maintained within an other sheath. In another preferred embodiment, one or more ends of clip 650 are trumpet shaped. In another preferred embodiment, clip 650 is designed to have minimal contact with artery and/or vein intima. In another preferred embodiment, one or more ends of clip 650 traverses more than ninety degrees from its central axis to form a doughnut or toroid shape. Note that this particular embodiment accommodates greater distance between vessel walls, and requires less precision in placement.
In another embodiment, clip 650 has an inner lumen between one and fifteen millimeters when expanded, and in a preferred embodiment the inner lumen is between two and ten millimeters when expanded. In another embodiment, clip 650 has an integrated sensor, not shown, that can be used to measure flow control or flow information, such as through the use of imaging or via embedded electronics wirelessly transmitted to an outside receiver.
It is important that anastomotic clip 650 not migrate after it is placed between artery 630 and vein 620. In order to maintain a secure placement, fixation means can be used, not shown, including but not limited to one or more of: suture, staples, clips and biocompatible glues.
Referring now to
Anastomosis 711 of
Various cardiac veins, or vein locations can be used in the method of
Other embodiments of the invention will be apparent to those skilled in the art from consideration of the specification and practice of the invention disclosed herein. It is intended that the specification and examples be considered as exemplary only, with a true scope and spirit of the invention being indicated by the following claims.